automatically capture last mutating systemd unit, highlight as "agent"

This is very similar to https://github.com/coreos/rpm-ostree/pull/1368/commits/83c942dac2fec09e5077b601438229f7da8e6c33 and later https://github.com/coreos/rpm-ostree/pull/2459

Basically...it'd be really useful to be able to see from `bootc status` the basic information about which calling process (systemd unit e.g.) last mutated the system. Maybe even a history of such changes?

And perhaps like the rpm-ostree change there should be an explicit API to claim "I am the active driver" and require disabling that active driver to make other changes.

An architectural thing to debate here is whether this history canonically lives in the journal or whether it's bound e.g. to deployment metadata. (I think probably the latter, it's a tiny amount of information and some edge systems e.g. will want to turn the local journal off to reduce persistent writes, only doing select remote logging).

I would also really like to figure out how to better wire up our default simplistic systemd unit+timer into this.